1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser beam printer and a copying machine, for forming an image on a recording medium, and more particularly to an image forming apparatus using a jumping development process to develop an electrostatic latent image formed on a photosensitive member.
2. Description of the Related Art
An image forming apparatus such as a laser beam printer and a copying machine is usually provided with a photosensitive member and an exposure unit such as a laser scanner for exposing the photosensitive member to light. In the vicinity of the surface of the photosensitive member there are provided a developing unit for developing an electrostatic latent image formed on the photosensitive member and a transferring unit for transferring a toner image obtained by development of the electrostatic latent image onto a recording paper. Further, a fixing unit for fixing the toner image transferred onto the recording paper is provided near a paper outlet from which the recording paper after printed is ejected.
A development system for developing the electrostatic latent image formed on the photosensitive member in such an image forming apparatus is generally classified into a one-component development system and a two-component development system. The one-component development system is simple in mechanism; so, it is applied especially to a compact laser beam printer.
FIG. 8 shows the principle of operation of a jumping development process as one process in the category of the one-component development system. As shown in FIG. 8, a photosensitive drum 11 and a conductive sleeve 12 are arranged in such a manner that their axes of rotation are parallel to each other and their cylindrical surfaces are spaced from each other. A toner 14 is supplied from a toner supply section (not shown) to the conductive sleeve 12. The toner 14 contains magnetic powder in about a half proportion, and it is therefore magnetically attracted to the conductive sleeve 12 formed of magnets. The toner 14 attracted to the conductive sleeve 12 is carried to a position opposed to the photosensitive drum 11 by rotation of the conductive sleeve 12 in a direction depicted by an arrow P. During this course, the toner 14 on the conductive sleeve 12 is scraped to form a thin toner layer 15 by a magnetic blade 13 provided in the vicinity of the conductive sleeve 12. At this time, particles of the toner 14 are vigorously rubbed with each other or with the surface of the conductive sleeve 12 with the result that the particles are negatively charged. Thus, the thin toner layer 15 negatively charged is formed on the conductive sleeve 12 at a portion thereof opposed to the photosensitive drum 11.
Prior to exposure, the photosensitive drum 11 is uniformly negatively charged by a charge corotron (not shown). As being rotated in a direction depicted by an arrow Q, the surface of the photosensitive drum 11 is sequentially exposed to light according to image information. Owing to this exposure according to the image information, an exposed area on the surface of the photosensitive drum 11 is neutralized in electric charge, but an unexposed area on the surface of the photosensitive drum 11 remains negative in electric charge. Accordingly, an electrostatic latent image 16 is formed on the exposed area on the surface of the photosensitive drum 11. At this time, the unexposed area on the photosensitive drum 11 has a potential of about -300 V and the exposed area on the photosensitive drum 11 has a potential of several tens of minus volts.
A developing bias is applied to the conductive sleeve 12, so as to make the negatively charged toner jump to the photosensitive drum 11. The developing bias is obtained by superimposing a DC voltage component generated from a DC voltage generating circuit 17 and an AC voltage component generated from an AC voltage generating circuit 18. The AC voltage component is applied for the purpose of always reciprocating the toner between the conductive sleeve 12 and the photosensitive drum 11, which is characteristic of the jumping development process. On the other hand, the DC voltage component is applied for the purpose of making the toner 14 supplied to the photosensitive drum 11 by the AC voltage component cling to to the exposed area on the photosensitive drum 11, thereby forming a toner image on the exposed area.
FIG. 9 shows a typical waveform of a developing bias 31 in the jumping development process. A DC voltage component 32 of the developing bias 31 is set at a potential of about -200 V corresponding to an intermediate potential between a potential 33 in the unexposed area of the photosensitive drum 11 and a potential 34 in the exposed area of the photosensitive drum 11. An AC voltage component of the developing bias 1 is represented by a sine wave having an amplitude of about 2 kV and a frequency of several kilohertz. In the phases shown by hatched areas in FIG. 9, the potential of the developing bias 31 is lower than the surface potential of the photosensitive drum 11, and the toner has jumped to the photosensitive drum 11. Conversely, in the phase where the potential of the developing bias 31 is an enough high positive value, the toner has returned from the photosensitive drum 11 to the developing unit.
FIG. 10 shows a conventional developing bias generating section for generating such a developing bias. The developing bias generating section includes a CPU (Central Processing Unit) 51 for controlling on/off switching of the developing bias and an oscillating circuit 52 adapted to oscillate under control of the CPU 51.
The CPU 51 generates a developing bias on/off signal 53 to an output controller 54 at a given timing according to a program stored in a memory (not shown). The output controller 54 includes a power supply for oscillating the oscillating circuit 52, and switches the power supply according to the developing bias on/off signal 53 output from the CPU 51.
The oscillating circuit 52 is composed of a capacitor 55, a primary wiring inductance of a transformer 56, and a transistor 57. When receiving an on-signal from the CPU 51, the output controller 54 switches on the power supply to set given potentials at output terminals 58 and 59, thereby starting oscillation of the oscillating circuit 52. Conversely, when receiving an off-signal from the CPU 51, the output controller 54 switches off the power supply to stop the oscillation of the oscillating circuit 52.
A capacitor 60 and a diode 61 are connected to a secondary wiring of the transformer 56, and resistors 62 and 63 are also connected to form a closed circuit. When the oscillating circuit 52 starts oscillating, electric charges are stored into the capacitor 60. An electric current is allowed to flow only clockwise in the closed circuit through the diode 61, so that a potential of a developing bias 64 is shifted in the negative direction by the electric charges stored in the capacitor 60. In other words, a negative DC voltage component is generated by the capacitor 60. Thus, the DC voltage component is superimposed with an AC voltage component to obtain the developing bias 64.
While a semiconductor laser is used as a light source for exposing the photosensitive drum, an output level of the semiconductor laser greatly fluctuates because of a change in ambient temperature or an aged factor. To compensate such a fluctuation in output level of the semiconductor laser, feedback control is usually performed. In the feedback control, the semiconductor laser is driven by a given drive current for adjustment and a gain of a drive circuit is so adjusted as to make the output constant. Such a light intensity adjustment is periodically performed to thereby maintain the output level of the semiconductor laser always at a constant value.
The light intensity adjustment as mentioned above is inhibited while the photosensitive drum is being exposed according to image information. There is disclosed in Japanese Patent Laid-open No. 2-134656 an image forming apparatus wherein the light intensity adjustment is performed in an area on a photosensitive member to be scanned by a laser beam output from the semiconductor laser during a time period when the scanning is suspended. In such an image forming apparatus, an intended reduction in print time brings about insufficiency of time for the light intensity adjustment. Accordingly, it is difficult to maintain an image quality.
In contrast, there is disclosed in Japanese Patent Laid-open No. 2-131261 an electrophotographic printer wherein the light intensity adjustment is performed at a given timing between the exposures according to image information. In such an electrophotographic printer, a meaningless electrostatic latent image unrelated to the image information is undesirably formed on the photosensitive drum by the adjusting light from the semiconductor laser. If the meaningless electrostatic latent image is developed, not only the toner is wastefully consumed, but also a transferring unit is stained with the toner. Further, in an image forming apparatus adopting a constant transfer system, a back surface of a recording paper is stained. To solve this problem, the electrophotographic printer disclosed in Japanese Patent Laid-open No. 2-131261 mentioned above controls the potential of the developing bias to a given value at which the meaningless electrostatic latent image cannot be developed during a time period when the meaningless electrostatic latent image passes a position opposed to the developing unit.
Thus in the conventional image forming apparatus adapted to perform the light intensity adjustment during a time period between the exposures according to image information, development of the meaningless electrostatic latent image unrelated to the image information can be prevented by controlling the potential of the developing bias.
FIGS. 11A to 11C show various signal waveforms when the developing bias is turned off irrespective of the phase of the AC voltage component as in the related art. More specifically, FIG. 11A shows a waveform of the developing bias, in which hatched areas represent the phases where the toner has jumped to the photosensitive drum. FIG. 11B shows a developing bias on/off signal, in which when this signal is H (high) level, the developing bias is on whereas when this signal is L (low) level, the developing bias is off. FIG. 11C shows an output signal from the semiconductor laser, in which when this signal is H level, the semiconductor laser is on.
Initially, the developing bias same as that shown in FIG. 9 is applied to the conductive sleeve. In this condition, the semiconductor laser remains off, and the photosensitive drum is therefore unexposed. When performing the light intensity adjustment, the CPU changes the developing bias on/off signal from H level to L level, thereby turning off the developing bias. In the case shown in FIGS. 11A to 11C, the developing bias is turned off in the phase represented by the rightmost hatched area, that is, in the phase where the toner has jumped to the photosensitive drum but has not yet returned to the developing unit. In other words, the toner is left on the photosensitive drum in this phase. Thus, when the developing bias is turned off irrespective of the phase of the AC voltage component, there is a possibility that the toner may be left on the photosensitive drum. As a result, there remains the above problem of wasteful consumption of the toner and staining of the transferring unit. Such a problem occurs not only when the light intensity adjustment is performed but also whenever the developing bias is turned off.